Display device with a channel plate and a cross bar electrode addressing system

ABSTRACT

A display device including an electron tube for image reproduction comprising an electron source having a substantially flat emissive surface and comprising a display screen discrete points of which are caused to luminesce by electrons impacting upon a luminescent substance. The selection of the points to be caused to luminesce is performed with the aid of two grids arranged between the electron source and the display screen, each of which have non-intersecting electrodes, a so-called cross-bar. The surface of at least one grid facing the electron source is provided with an insulating or poorly conductive plate having closely adjacent, substantially parallel, narrow passages. The passages have conductive surfaces. The surface of the plate in which the passages are opening out and on which no grid is provided is covered with an electrically conductive layer. The passages are, in particular, secondary-emissive.

United States Patent [191 Johanns et a1.

all of Emmasingel, Eindhoven, Netherlands U.S. Philips Corporation, New York, NY.

Filed: Mar. 1, 1972 Appl. No.: 230,982

Related U.S. Application Data Continuation of Ser. No. 881,692, Dec. 3, 1969, abandoned.

Assignee:

[30] Foreign Application Priority Data Dec. 14, 1968 Netherlands 6818016 U.S. Cl 313/105, 313/95 Int. Cl. I-I0lj 43/22 Field of Search 313/95, 105, 103, 104

[56] References Cited UNITED STATES PATENTS 6/1951 Toulon 313/2 X 2/1959 McGee 313/95 3/1959 Riggen .1313/102 2/1961 Michlin ..313/73x 1 June 25, 1974 Primary Examiner-l-lerman Karl Saalbach Assistant ExaminerSiegfried l-l. Grimm Attorney, Agent, or FirmFrank R. Trifari; Carl P. Steinhauser [5 7] ABSTRACT A display device including an electron tube for image reproduction comprising an electron source having a substantially flat emissive surface and comprising a display screen discrete points of which are caused to luminesce by electrons impacting upon a luminescent substance. The selection of the points to be caused to luminesce is performed with the aid of two grids arranged between the electron source and the display screen, each of which have non-intersecting electrodes, a so-called cross-bar. The surface of at least one grid facing the electron source is provided with an insulating or poorly conductive plate having closely adjacent, substantially parallel, narrow passages. The passages have conductive surfaces. The surface of the plate in which the passages are opening out and on which no grid is provided is covered with an electrically conductive layer. The passages are, in particular, secondary-emissive.

8 Claims, 1 Drawing Figure PATENTED JUN 2 5 I974 JOHANNES H.MJOWAW JOHANNES H.TH.VAN ROOSMALEN THUS J. DE BQ DISPLAY DEVICE WITH A CHANNEL PLATE AND A CROSS BAR ELECTRODE ADDRESSING SYSTEM This is a continuation, of application Ser. No. 881,692, filed 12-3-69 now abandoned.

The invention relates to an electron discharge tube for image reproduction, comprising an electron source having a substantially uniformly emissive surface. This tube also comprises a substantially flat grid having only non-intersecting electrodes, and a second grid substantially parallel to said first grid only having nonintersecting electrodes, each of which crosses all electrodes of the first grid. The tube also has a display screen further remote from the electron source while on the surface of at least one grid facing the electron source an insulating or poorly conductive plate is provided having closely adjacent, substantially parallel, narrow passages opening out in said surface.

Such a tube, in which for example the electrodes of one grid extend all in parallel and the electrodes of the first grid and those of the second grid cross each other at right angles, has the advantage as compared with a conventional cathode-ray tube that the dimension at right angles to the display screen is considerably smaller and a large image surface can be obtained. in an embodiment disclosed in US. Pat. No. 2,558,019 the electron-emitting surface of the electron source is substantially equal to the surface of the display screen and by the selection of the voltage of the electrodes of the first grid and of the electrodes of the second grid discrete points of the display screen are caused to luminesce by the electron impact on a phosphor. The electrodes of a grid have either a given low voltage or a given high voltage, the electrons not being passed in the case of, the low voltage and the electrons being passed in the case of the high voltage.

It has been determined that if no insulating or poorly conductive, plate is present on the surface of a grid facing the electron source, such a structure gives rise to problems when various electrodes of the said grid are simultaneously at the high voltage. On the side of the grid facing the electron source the electrons are concentrated in the vicinity of the electrodes of the grid at the high voltage and even electrons emanating from a region located nearer an electrode at the low voltage. At the place of an electrode at the high voltage located between two electrodes at the low voltage more electrons are passed than at the place of an electrode at the high voltage located between two electrodes at the high voltage. This results in that the perceptible aspect of an image element of a display screen is also determined by the environments of this image element, which becomes manifest in the brightness. If an insulating or poorly conductive plate is present on the surface of a grid facing the electron source, this problem is obviated. The electrons going out from a given part of the electron source can strike only a-given part of the display screen; a further part of the display screen cannot be struck by these electrons. It is thus ensured that the brightness of an image point is not influenced by the simultaneous luminescence or nonluminescence of surrounding image points. The selection takes place in this case by the fact that the potential of a given electrode of a grid is so low that the kinetic energy of the electrons is not sufficient to compensate for the potential difference between the electrons in the passage concemed and the electrode so that substantially no electrons emanate from these passages. The potential of a further electrode is higher and the electrons emanate from the relevant passages.

However, such a structure gives rise to problems in that the length of the passages has to satisfy definite requirements; for a satisfactory selection a given length is required, but the passages must not be too long because otherwise the charge of the walls of the passages would play a given part or electrons would be intercepted at the walls of the passages. The invention obviates this disadvantage.

According to the invention the passages of the plate have conductive surfaces, while the surfaces of the plate in which the passages open out and on which no grid is provided are covered with an electric conductor. In this case the passages may be longer because with the application of a voltage to the conductor on the one hand and to the grid on the other hand the potential at the walls is defined; the walls are then not charged and electrons are less readily intercepted at the walls. If the plate is an electrically insulating plate, the walls of the passages are provided with a conductive coating. In the case of a poorly conductive plate the passages usually have by nature a sufiiciently conductive surface. The electrodes of a grid may be separate wires. As an alternative, the grid may be formed by applying conductors to the plate, for example, by vapour-deposition. The number of passages in the direction at right angles to the direction of length of the electrodes of a grid is at least equal to the number of electrodes.

If in both grids different electrodes are simultaneously at the high voltage, two insulating or poorly conductive plates are provided. Therefore, more particularly on the surface of the first grid facing the electron source a first insulating or poorly conductive plate is provided having closely adjacent, substantially parallel, narrow passages opening out in said surface, while on the surface of the second grid facing the electron source a second insulating or poorly conductive plate is provided having closely adjacent, substantially parallel, narrow passages opening out in said surface. The passages in the plates moreover, have conductive surfaces and the surfaces of the plates in which the passages open out and on which no grid is provided are covered with an electric conductor. In this case of two plates the passages in the first plate need not be parallel to those in the second plate.

The passages also may have secondary emissive surfaces. If a sufficiently high voltage difference prevails across passage, the voltage of the electrode on the side of the electron source being lower than the voltage of the electrode on the other side of the passage, this passage operates by secondary emission as an electron multiplier for the incoming electrons. This is advantageous because the points of the display screen are then struck by a greater number of electrons. The velocity of the electrons required to cause the display screen to luminesce by their impact at the relevant point may then be lower. The selection is performed in this case also by the fact that for the electrons not allowed to pass the voltage difference across the relevant passages is so low that no muliplication takes place; as the case may be, the voltage difference across these passages may even be negative.

In the second plate, in principle, only the passages se lected by the voltage of the electrodes of the first grid receive electrons. This selection is in practice not perfect and a single electron nevertheless enters other passages. In order to avoid such an electron in the passage operating as an electron multiplier giving rise to a great number of electrons on the exit side of the second plate, it is important that the voltage difference across this passage should be considerably lower than that at which saturation occurs. If no saturation occurs, the ratio between the number of incoming electrons and the number of outgoing electrons for passages of the same dimensions is approximately the same, while in a first instance the multiplication depends upon the ratio between the length and the diameter of the passages. With an increase in the voltage difference across a passage the multiplication initially increases, but at a given value of the voltage difference, which depends upon the dimensions of the passages, the electric resistance of the plate and the number of incoming electrons, a maximum value of multiplication is attained, which varies only slightly with a further increase in the voltage difference. If the voltage difference is higher than that at which the maximum value of multiplication is attained, the number of outgoing electrons is no longer a function of the number of incoming electrons and is substantially constant with passages of the same lengthto-diameter ratio. Therefore, in the present case the voltage difference across the passages must be considerably lower than that at which saturation occurs.

The electron source has a substantially uniformly emissive surface. It may be formed by a uniformly exposed photocathode. It is preferably constructed so that its part located at the grid side is formed by a plate having closely adjacent, substantially parallel, narrow passages of substantially the same diameter, the two substantially parallel surfaces of said plate in which the passages open out, being covered by electric conductors, said passages having secondary-emissive and conductive surfaces. If this plate is operating in the saturated state, which means that the voltage difference across the plate exceeds that at which the maximum value of multiplication is attained, the electron source emits electrons uniformly. The kinetic energy with which the electrons are going out from the electron source depends upon the distance between the place of the last collision on the wall of a passage in the plate and the exit surface of said plate and this energy is the higher, the larger is said distance. This gives rise to a spread in the kinetic energy of the electrons going out from the electron source. This spread may be reduced by certain measures. For this purpose an additional plate may be provided at the screen side of the plate of the electron source, which plate also has passages in which secondary emission occurs. The plate of the electron source and the additional plate may be united, while the conductor is common to both. In order to ensure little spread in kinetic energy of the electrons emanating from the additional plate the voltage difference across the additional plate has to be as low as possible, whereas the plate operates nevertheless as an electron multiplier. It is furthermore ensured that the electrons entering the passages of the additional plate collide at least once on the wall of a passage, for example, by the choice of the angle between the axes of the passages in the plate of theelectron source and those of the additonal plate. Since the number of collisions of the electrons on the walls of the passages is fixed, the number per passage is approximately constant. The distance between the place of the last collision on the wall of a passage in the additional plate and the exit surface of the additional plate is then not much different for the various passages so that a comparatively low spread of kinetic energy of the electrons going out from the additional plate is ensured. This additional plate and the first plate, on which the first grid is provided, may in a given case be united, while the conductor is common to both of them. Moreover, the first plate may fulfil the function of the additional plate for the electron source. The electron source is therefore in particular provided on the side of the grids with a plate having closely adjacent, substantially parallel, narrow passages of substantially the same diameter, whose two substantially paral lel surfaces in which the passages open out are covered with electric conductors. These passages have secondary-emissive and conductive surfaces, while between the electron source and the first grid at least one plate is arranged, which is provided with closely adjacent, substantially parallel narrow passages. The surface of this plate located on the side of the electron source has passages which open out in it having an electric conductor in common with the electron source. The first grid is arranged on the surface lying on the side of the first grid, where the passages open out and any possible further surfaces in which the passages open out are covered with electric conductors which are common to consecutive plates.

In the plane of the second grid electrons emanate from those passages which correspond to the points of the display screen which must luminesce. For this purpose these electrons, have to impinge on the display screen with adequate velocity, for which purpose they are accelerated between the second grid and the display screen. The same brightness may, however, also be obtained by a greater number of electrons striking the display screen with lower velocity. It is therefore advantageous to produce a multiplication after the second grid and this is preferred over the use of an electron source emitting a greater number of electrons because in said case the larger electron flow would also be present at the place of selection, which would render the selection more difficult. The so-called postmultiplication is obtained by arranging a further plate having secondary-emissive passages between the second grid and the display screen and conductors on each side thereof. Then a plate is provided between the second grid and the display screen, which plate has closely adjacent, substantially parallel, narrow passages, the two surfaces in which the passages open out, being covered with electric conductors, said passages having secondary-emissive and conductive surfaces. With a comparatively large distance between the second grid and the post-multiplying plate the shape of the field between the electrodes of the second grid and the conductor on the post-multiplying plate does not affect substantially the direction of travel of the electrons. With a comparatively small distance, however, crosstalk may occur. In order to avoid this and also for structural reasons it is adviseable to unite the second plate and the post-multiplying plate. This is, however, not possible in all cases without the need for further means because the second plate comprises a grid with electrodes and the post-multiplying plate is provided basically with an electric conductor on that side. In certain cases it is permissible for the conductor on the postmultiplying plate to be a grid, that is to say, when this plate does not operate in the saturated state. In this case the brightness variations of the displayed image may be obtained by varying the voltages between the electrodes of the various grids. The second grid is then arranged in particular on the surface of the plate lying between the second grid and the display screen. If the postmultiplying plate is operating in the saturated state, in which case brightness variations of the displayed image are obtained by the control of the time during which the electrons are allowed to pass, the presence of the second grid on the post-multiplying plate gives rise to difficulties, because the electrodes of the second grid have to be switched over a much too large voltage range. In order to avoid these difficulties an insulating or poorly conductive plate is arranged between the second plate and the post-multiplying plate, one side of said plate having the second grid and the other side being united with the postmultiplying plate. Therefore, in particular at least one insulating or poorly conductive plate is arranged between the second grid and the plate provided between the second grid and the display screen. This plate has closely adjacent, substantially parallel narrow passages, while the second grid is arranged on the surface lying on the side of the second grid, where the passages open out. The further surfaces in which the passages open out are covered with electric conductors which are common to consecutive plates.

The invention will be described more fully with reference to an embodiment shown in a drawing, which is a schematic sectional view of an electron tube, the various dimensions of which are not shown to the same scale.

Referring to the FIGURE, reference numeral 1 designates a glass envelope, which accommodates a number i of interconnected glass plates 5, 7, 9, l1 and 13 having closely adjacent, narrow passages, the surfaces of each plate where the passages open out being provided with conductors. For the sake of clarity only a few passages are shown not to scale. A conductive layer 2, a porous insulating layer 3, a conductive layer 4, the plate 5 and a conductor 6 form an electron source. The passages in the plate 5 are at an angle of 15 to the axis of the tube. The passages in the plates 7, 9, 11 and 13 are parallel to the tube axis. The plates 5 and 7 have the conductor 6 in common. The plates 7 and 9 have in common a grid 8 consisting of a plurality of electrodes, the direction of length of which is parallel to the plate of the drawing. The plates 9 and l 1 have in common a grid 10 consisting of a plurality of electrodes whose direction of length is at right angles to the plane of the drawing. The plates 11 and 13 have in common a conductor 12 and the other side of the plate 13 is provided with a conductive layer 14. Part of the envelope 1 is covered with a display screen comprising a phosphor layer 15 and a thin metal layer 16, for example, of aluminum. in one case the plates 5 and 13 have a thickness of 1 cm and the plates 7, 9 and 11 a thickness of 1 mm. The passages in the plates 5 and 13 have a diameter of 200 p. and those in the plates 7, 9 and 11 a diameter of 100 u. The walls of the passages of the plates are rendered conductive by a reduction treatment to such an extent that in the plates 5 and 13 the resistance of 1 square cm of the plate surface is 500 MOhms, whereas this value in the plates 7, 9 and 11 is MOhms. The walls of the passages have a secondary-emission coefficient of 3 to 4 at perpendicular electron incidence. The conductive layer 2 of a thickness of 1 mm consists of aluminum, the layer 3 of a thickness of a few microns consists of A1 0 and the conductive layers 4 and 14 comprise each a chromium-nickel layer and a gold layer. The electric conductivity of gold exceeds that of chromiumnickel, but gold does not adhere as well to glass as chromium-nickel so that this combination is advantageous. The conductors 6 and 12 are formed by a conductor on one plate comprising a chromium-nickel layer and a gold layer and a conductor on the other plate constructed in the same manner. The grids 8 and 10 are formed by platinum wires of a thickness of 0.1 mm. The layer 2 is at a voltage of 1,600 V, the layer 4 at a voltage of l,500 V and the conductor 6 at a voltage of 0 V. With this voltage difference of 1,500 V across the plate 5 and with a layer of Al O of a thickness of a few microns, across which .a voltage difference of V is present, the plate 5 having said resistance value and a length-to-diameter ratio of the passages of 50/cos 15 52 operates as an electron multiplier in the saturated state and the plate 5 has a substantially flat emissive surface, the electrons being at a potential of 0 V. Their velocity depends upon the distance between the place of the last collision on the wall of a passage and the layer 6.

The electrodes of the grid 8 are at a voltage of either 50 V or 50 V. With the passages of the plate 7 associated with electrodes of the grid 8 at a voltage of 50 V, across the passages thus a voltage difference of 50 V, being present, the plate having said resistance value and a length-to-diameter ratio of the passages of 10 operates as an electron multiplier not being saturated. The number of electrons going out from each passage is then substantially proportional to the number of electrons entering each passage; in connection with the operation described for the plate 5 this number of electrons entering in each passage is substantially the same so that also the number of electrons emerging per passage is substantially the same. Because the passages in the plate 7 are at an angle of 15 to the passages in the plate 5, there is substantially no spread in the kinetic energy of the outgoing electrons. The voltage difference across the passages associated with the'other electrodes of the grid 8 at a voltage of 50 V is 50 V and as a result thereof no electrons emanate. The electrons emanating at the place of the electrodes of the grid 8 at a voltage of 50 V originate only from the corresponding parts of the emissive surface of the electron source.

The electrodes of the grid 10 are at a voltage of either -50 V or 50 V. To the passages of the plate 9, where electrons from the plate 7 enter and which are associated with electrodes of the grid 10 at the voltage of 50 V it follows that electrons emanate. In this case in connection with the voltage difference of 0 V no electron multiplication occurs. To the passages of the plate 9, where electrons from the plate 7 enter and which are associated with electrodes of the grid 10 at the voltage of 50 V it applies that no electrons emanate because their kinetic energy is not sufficient for overcoming the voltage difference of -100 V, while electron multiplication is quite out of the question. It should be noted that a voltage difference of 0 or 100 V prevails across the passages in the plate 9, where in principle no electrons from the plate 7 enter, in accordance with the voltage of the electrodes of the grid 10. Even with the highest value the plate 9 having said resistance value and a length'to-diameter ratio of the passages of does not operate in the saturated state; if nevertheless a single electron should enter a passage of the plate 9 associated with an electrode of grid 8 at a voltage of 50 V, the number increased by the multiplication is still very small as compared with the increased number of selected electrons.

The conductor 12 is at a voltage of 100 V. Across the passages of the plate 11, which receive electrons from the plate 9 and which are therefore associated with electrodes of the grid 10 at the voltage of 50 V, 21 voltage difference of 50 V is present. With said resistance value and a length-to-diameter ratio of the passages of 10 the plate 11 then operates as an electron-multiplier not being saturated. In a similar manner as explained for the plate 7 a number of electrons goes out from said passages which is approximately the same for each passage. It should be noted that across the passages of the plate 11 associated with the electrodes of the grid 10 at a voltage of 50 V a voltage difference of 150 V is present. Also with this voltage the plate 11 does not operate in the saturated state; if nevertheless a single electron should go out from a passage of the plate 9 associated with an electrode of the grid 10 at the voltage of -50 V, the number increased by multiplication in the plate is still very small as compared with the increased number of selected electrons.

The conductive layer 14 is at a voltage of 1,100 V. With the voltage difference of 1,000 V across the plate 13, the latter having said resistance value and a lengthto-diameter ratio of the passages of 50 operates as an electron-multiplier not being saturated. Thus, postmultiplication occurs in the passages of the plate 13 so that the number of electrons at the selected places is increased. This increased number of electrons of a potential of 1,100 V is accelerated towards the display screen the aluminum layer 16 of which is at a voltage of 15,000 V. Thus, discrete points of the phosphor layer are caused to luminesce by electrons emanating from corresponding parts of the electron source.

It is thus possible to reproduce characters and digits with great brightness and high resolving power. It is furthermore possible to construct in this manner display panels for television purposes.

What is claimed is:

1. An electron discharge tube for image reproduction comprising an envelope, and within said envelope an electron source having a substantially uniformly emissive surface, a substantially flat first grid having a plurality of parallel conductors spaced from said electron source, a second grid spaced from said first grid and having a plurality of parallel conductors each of which forms a common angle with the conductors of the first grid without intersecting those of said first grid, a display screen further remote from the electron source, and at least a first substantially insulating plate on the surface facing the electron source of at least one grid, said plate having closely adjacent substantially parallel 8 narrow passages bounded by walls having conductive layers thereon, said passages opening out on both surfaces of the plate, and an electric conductor covering the surface of the plate remote from said grid in which the passages open out.

2. An electron discharge tube as claimed in claim 1 in which said first substantially insulating plate is provided on the surface of the first grid, and a second substantially insulating plate is provided having closely ad- 10 jacent, substantially parallel narrow passages bounded by walls having conductive layers thereon, said latter passages opening out on a surface of the second grid facing the electron source, the surface of the second plate in which the passages open out remote from the second grid being covered with an electric conductor.

3. An electron discharge tube as claimed in claim 2 wherein the first grid is secured to the surface of the second substantially insulating plate facing the electron source.

4. An electron discharge tube as claimed in claim 1 wherein the walls bounding the passages of said insulating plate are secondary-emissive.

5. An electron discharge tube as claimed in claim 1 wherein the electron source comprises a further plate adjacent the first grid having closely adjacent, substantially parallel, narrow passages of substantially the same diameter, said further plate having two substantially parallel surfaces in which the passages open out and which are covered with electric conductors, said passages being bounded by walls having secondaryemissive and conductive layers, said first substantially insulating plate being provided between the electron source and the first grid, and an electric conductor in common with the electron source on the surface of said first plate facing the electron source, said first grid being secured to the surface of the first plate remote from the electron source.

6. An electron discharge tube as claimed in claim 1 wherein between the second grid and the display screen a second plate is provided which has closely adjacent, substantially parallel, narrow passages and the two surfaces on which the passages open out are covered with electric conductors, said passages being bounded by walls having secondary-emissive and conductive layers thereon.

7. An electron discharge tube as claimed in claim 6 wherein a post-multiplication plate having secondaryemissive passages is disposed between the second grid and the display screen, said post-multiplication plate being secured to said second plate.

8. An electron discharge tube as claimed in claim 7 wherein the second grid is secured to one surface of the second plate remote from the display screen, and said post-muliplication plate being secured to the opposite surface of said secondv plate.

72 33? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,91 9 mud June; 25. 1974 ven or) Johannes H.M. Johanns et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

- Title page: "Johannes Hendricus Van Roosmalen" should read -Johannes Hendrikus 'Theodorus Van Roosmalen- Column 3,' line 4, after "multiplier" insert -and-.

Signed and sealed this 5th day of November 1974.

(SEAL) Attest:

McCOY M; mason JR. c. MARSHALL DANN Attesting Officer Comissioner of Patents 

1. An electron discharge tube for image reproduction comprising an envelope, and within said envelope an electron source having a substantially uniformly emissive surface, a substantially flat first grid having a plurality of parallel conductors spaced from said electron source, a second grid spaced from said first grid and having a plurality of parallel conductors each of which forms a common angle with the conductors of the first grid without intersecting those of said first grid, a display screen further remote from the electron source, and at least a first substantially insulating plate on the surface facing the electron source of at least one grid, said plate having closely adjacent substantially parallel narrow passages bounded by walls having conductive layers thereon, said passages opening out on both surfaces of the plate, and an electric conductor covering the surface of the plate remote from said grid in which the passages open out.
 2. An electron discharge tube as claimed in claim 1 in which said first substantially insulating plate is provided on the surface of the first grid, and a second substantially insulating plate is provided having closely adjacent, substantially parallel narrow passages bounded by walls having conductive layers thereon, said latter passages opening out on a surface of the second grid facing the electron source, the surface of the second plate in which the passages open out remote from the second grid being covered with an electric conductor.
 3. An electron discharge tube as claimed in claim 2 wherein the first grid is secured to the surface of the second substantially insulating plate facing the electron source.
 4. An electron discharge tube as claimed in claim 1 wherein the walls bounding the passages of said insulating plate are secondary-emissive.
 5. An electRon discharge tube as claimed in claim 1 wherein the electron source comprises a further plate adjacent the first grid having closely adjacent, substantially parallel, narrow passages of substantially the same diameter, said further plate having two substantially parallel surfaces in which the passages open out and which are covered with electric conductors, said passages being bounded by walls having secondary-emissive and conductive layers, said first substantially insulating plate being provided between the electron source and the first grid, and an electric conductor in common with the electron source on the surface of said first plate facing the electron source, said first grid being secured to the surface of the first plate remote from the electron source.
 6. An electron discharge tube as claimed in claim 1 wherein between the second grid and the display screen a second plate is provided which has closely adjacent, substantially parallel, narrow passages and the two surfaces on which the passages open out are covered with electric conductors, said passages being bounded by walls having secondary-emissive and conductive layers thereon.
 7. An electron discharge tube as claimed in claim 6 wherein a post-multiplication plate having secondary-emissive passages is disposed between the second grid and the display screen, said post-multiplication plate being secured to said second plate.
 8. An electron discharge tube as claimed in claim 7 wherein the second grid is secured to one surface of the second plate remote from the display screen, and said post-muliplication plate being secured to the opposite surface of said second plate. 